Histamine [2-(imidazol-4-yl)ethylamine] is a transmitter substance. Histamine exerts a physiological effect via multiple distinct G-protein coupled receptors. It plays a role in immediate hypersensitivity reactions and is released from mast cells following antigen IgE antibody interaction. The actions of released histamine on the vasculature and smooth muscle system account for the symptoms of the allergic response. These actions occur at the H1 receptor (Ash, A. S. F. and Schild, H. O., Br. J. Pharmac. Chemother. 1966, 27:427-439) and are blocked by the classical antihistamines (e.g. diphenhydramine). Histamine is also an important regulator of gastric acid secretion through its action on parietal cells. These effects of histamine are mediated via the H2 receptor (Black, J. W. et al., Nature 1972, 236:385-390) and are blocked by H2 receptor antagonists (e.g. cimetidine). The third histamine receptor —H3— was first described as a presynaptic autoreceptor in the central nervous system (CNS) (Arrang, J.-M. et al., Nature 1983, 302:832-837) controlling the synthesis and release of histamine. Recent evidence has emerged showing that the H3 receptors are also located presynaptically as heteroreceptors on serotonergic, noradrenergic, dopaminergic, cholinergic, and GABAergic (gamma-aminobutyric acid containing) neurons. These H3 receptors have also recently been identified in peripheral tissues such as vascular smooth muscle. Consequently there are many potential therapeutic applications for histamine H3 agonists, antagonists, and inverse agonists. (See: “The Histamine H3 Receptor—A Target for New Drugs”, Leurs, R., and Timmerman, H., (Eds.), Elsevier, 1998; Morisset, S. et al., Nature 2000, 408:860-864.) A fourth histamine receptor —H4— was recently described by Oda, T. et al. (J. Biol. Chem. 2000, 275(47): 36781-36786).
The potential use of histamine H3 agonists in sleep/wake and arousal/vigilance disorders is suggested based on animal studies (Lin, J.-S. et al., Brain Res. 1990, 523:325-330; Monti, J. M. et al., Eur. J. Pharmacol. 1991, 205:283-287). Their use in the treatment of migraine has also been suggested (McLeod, R. L. et al., Soc. Neurosci. Abstr. 1996, 22:2010) based on their ability to inhibit neurogenic inflammation. Other applications could be a protective role in myocardial ischemia and hypertension where blockade of norepinephrine release is beneficial (Imamura, M. et al., J. Pharmacol. Exp. Ther. 1994, 271(3): 1259-1266). It has been suggested that histamine H3 agonists may be beneficial in asthma due to their ability to reduce non-adrenergic non-cholinergic (NANC) neurotransmission in airways and to reduce microvascular leakage (Ichinose, M. and Barnes, P. J., Eur. J. Pharmacol. 1989, 174:49-55).
Several indications for histamine H3 antagonists and inverse agonists have similarly been proposed based on animal pharmacology experiments with known histamine H3 antagonists (e.g. thioperamide). These include dementia, Alzheimer's disease (Panula, P. et al., Soc. Neurosci. Abstr. 1995, 21:1977), epilepsy (Yokoyama, H. et al., Eur. J. Pharmacol. 1993, 234:129-133), narcolepsy, eating disorders (Machidori, H. et al., Brain Res. 1992, 590:180-186), motion sickness, vertigo, attention deficit hyperactivity disorders (ADHD), learning and memory (Barnes, J. C. et al., Soc. Neurosci. Abstr. 1993, 19:1813), and schizophrenia (Schlicker, E. and Marr, I., Naunyn-Schmiedeberg's Arch. Pharmacol. 1996, 353:290-294). (Also see: Stark, H. et al., Drugs Future 1996, 21(5): 507-520; and Leurs, R. et al., Prog. Drug Res. 1995, 45:107-165 and references cited therein.) Histamine H3 antagonists, alone or in combination with a histamine H1 antagonist, are reported to be useful for the treatment of upper airway allergic response (U.S. Pat. Nos. 5,217,986; 5,352,707 and 5,869,479). Recently, a histamine H3 antagonist (GT-2331) was identified and is being developed by Gliatech Inc. (Gliatech Inc. Press Release Nov. 5, 1998; Bioworld Today, Mar. 2, 1999) for the treatment of CNS disorders.
As noted, the prior art related to histamine H3 ligands has been comprehensively reviewed (“The Histamine H3 Receptor-A Target for New Drugs”, Leurs, R., and Timmerman, H., (Eds.), Elsevier, 1998). Within this reference the medicinal chemistry of histamine H3 agonists and antagonists was reviewed (see: Krause, M. et al., and Phillips, J. G. and Ali, S. M., respectively). The importance of an imidazole moiety containing only a single substitution in the 4 position was noted together with the deleterious effects of additional substitution on activity. Particularly, methylation of the imidazole ring at any of the remaining unsubstituted positions was reported to strongly decrease activity. Additional publications support the hypothesis that an imidazole function is essential for high affinity histamine H3 receptor ligands (see: Ali, S. M. et al., J. Med. Chem. 1999, 42:903-909, and Stark, H. et al., and references cited therein). However many imidazole-containing compounds are substrates for histamine methyl transferase, the major histamine metabolizing enzyme in humans, which leads to shortened half-lives and lower bioavailability (see: Rouleau, A. et al., J. Pharmacol. Exp. Ther. 1997, 281(3): 1085-1094). In addition, imidazole-containing drugs, via their interaction with the cytochrome P450 monooxygenase system, can result in unfavorable biotransformations due to enzyme induction or enzyme inhibition (see: Kapetanovic, I. M. and Kupferberg, H. J., Drug Metab. Dispos. 1984, 12(5): 560-564; Sheets, J. J. and Mason, J. I., Drug Metab. Dispos. 1984, 12(5): 603-606; Back, D. J. and Tjia, J. F., Br. J. Pharmacol. 1985, 85:121-126; Lavrijsen, K. et al., Biochem. Pharmacol. 1986, 35(11): 1867-1878; Albengres, E. et al., Drug Safety 1998, 18(2): 83-97). The poor blood-brain barrier penetration of earlier histamine H3 receptor ligands may also be associated with the imidazole fragment (Ganellin, C. R. et al., Arch. Pharm. Pharm. Med. Chem. (Weinheim, Ger.) 1998, 331:395-404).
More recently, several publications have described histamine H3 ligands that do not contain an imidazole moiety, for example: Ganellin, C. R. et al.; Walczynski, K. et al., Arch. Pharm. Pharm. Med. Chem. (Weinheim, Ger.) 1999, 332:389-398; Walczynski, K. et al., Farmaco 1999, 54:684-694; Linney, I. D. et al., J. Med. Chem. 2000, 43:2362-2370; Tozer, M. J. and Kalindjian, S. B., Exp. Opin. Ther. Patents 2000, 10:1045-1055; U.S. Pat. No. 5,352,707; PCT Application WO 99/42458; PCT Application WO 02/076925; and EP Application 0978512, Feb. 9, 2000.
The compounds of the present invention do not contain the imidazole moiety, and its inherent liabilities, and yet maintain potency at the human H3 receptor as determined by receptor binding to the human histamine H3 receptor (see: Lovenberg, T. W. et al., Mol. Pharmacol. 1999, 55:1101-1107). Screening using the human receptor is particularly important for the identification of new therapies for the treatment of human disease. Conventional binding assays, for example, are determined using rat synaptosomes (Garbarg, M. et al., J. Pharmacol. Exp. Ther. 1992, 263(1): 304-310), rat cortical membranes (West, R. E. et al., Mol. Pharmacol. 1990, 38:610-613), and guinea pig brain (Korte, A. et al., Biochem. Biophys. Res. Commun. 1990, 168(3): 979-986). Only limited studies have been performed previously using human tissue but these allude to significant differences in the pharmacology of rodent and primate receptors (West, R. E. et al., Eur. J. Pharmacol. 1999, 377:233-239).
We now describe a series of phenylalkynes with the ability to modulate the activity of the histamine receptor, specifically the H3 receptor, without the inherent problems associated with the presence of an imidazolyl moiety.